In recent years, digital still cameras (hereinafter referred to simply as digital cameras) that enable input of picture image information, such as photographed landscapes and portraits, into a personal computer have rapidly become popular with the popularity of personal computers in households. Additionally, portable telephones that include portable cameras that incorporate compact image pickup modules with high functionality of the portable telephones have also rapidly become popular. Furthermore, including an image pickup module in compact information terminal equipment, such as PDAs, has also become popular.
In such devices that include an image pickup function, an image pickup element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), is used to provide the imaging function. Recently, great progress in the miniaturization of such image pickup elements has been achieved. This has resulted in a desire for the main body of such devices and the imaging lens system used in the imaging module to also be further miniaturized and made lighter. Additionally, image pickup elements with a larger number of pixels in the same area have been developed in order to achieve higher image quality, which requires higher resolution lens systems that are still very compact, as well as higher contrast performance.
Japanese Laid-Open Patent Application H10-048516 describes exemplary single focus lenses for such devices that include only three lens components, each of which is a lens element, with the image-side lens element having a biconvex shape and being immediately preceded on its object side by an aperture stop.
As stated above, recent image pickup elements are smaller and provide more pixels in a given detector area, which helps meet demands of higher resolution and more compactness that are especially required in imaging lenses for digital cameras. On the other hand, considerations of small cost and compactness have been the main considerations for imaging lenses for compact information terminal equipment, such as portable telephones with cameras. However, more recently, such devices have incorporated megapixel detectors (detectors that detect one million or more pixels), indicating increasing demand for higher performance in these devices as well, which has been accompanied by demands to make such devices smaller and to improve other performance properties. Therefore, development of various lenses in which the cost, performance, and compactness are comprehensively considered has been desired in order to respond to various user needs. For example, there is a need to develop a low cost, high performance image pickup lens that is suitable for digital cameras in the digital camera field as well as having satisfactory compactness for use in portable modular cameras.
In order to satisfy such requirements, for example, consideration has been given to using lenses with three or four lens components, each of which components may be a lens element, to achieve compactness and low cost, and using aspheric surfaces in order to achieve high performance. However, although aspheric surfaces assist in obtaining compactness and higher performance, they are unfavorable in terms of manufacturing considerations and rapidly increase costs of the lenses. Therefore, these issues must be considered in considering the use of aspheric surfaces. The single focus lenses described in Japanese Laid-Open Patent Application H10-048516 discussed above use three lens components that are lens elements and include aspheric surfaces, but the comprehensive performance of these lenses is unsatisfactory. For example, although the optical performance is adequate for some purposes, the lenses are not sufficiently compact. Generally, a lens with only three lens elements is insufficient in performance for a digital camera, even if it is sufficient in terms of performance for a portable modular camera. A lens with four lens elements can improve performance over a lens with three lens elements but it is unfavorable in terms of cost and compactness.